Understanding enzyme catalysis is one of the grand challenges of both experimental and computational biochemistry. The last decade has witnessed dramatic growth in both the range of application of, and methodological development within, the field of computational enzymology. Here we review applications of computational approaches based on quantum mechanics/molecular mechanics (QM/MM) methods to model a range of enzyme catalyzed reactions. We also highlight recent methodological advances in this field that point the way to increased accuracy, both via the use of ab initio quantum chemical methods that include the effects of electron correlation explicitly, and via the development of efficient sampling algorithms that allow for the calculation of reaction free energy profiles.